(1) Field of the Invention
The present invention relates to a fluorescent character display tube, and more particularly to a fluorescent character display tube in which the noise to occur at a frame portion of the tube is reduced.
(2) Description of the Related Art
A conventional fluorescent character display tube is shown by a plan view in FIG. 1 and a sectional view taken along line 2--2 of FIG. 1 is shown in FIG. 2. In FIG. 1, reference numeral 7 denotes a display pattern, 8 an anode terminal, 9 lead pin, and 13 a grid.
For purposes of maintaining a vacuum state, an anode base plate 5 and a cover glass (not shown) are assembled, and the resulting structure is sealed by a low melting point glass 1 applied on a spacer glass 6 as shown in FIG. 2. The spacer glass 6 and the anode base plate 5 sandwich a frame 2 on which a filament supporting member 3 is welded. In this configuration, there remains an airspace 14, though such an airspace is very slight, between the anode base plate 5 and the frame 2, and this airspace has presented difficulties in making the complete sealing. For this reason, as explained more fully later, the problem has often occurred wherein the frame and the filament supporting member vibrate resulting in noise.
Japanese Utility Model Application Kokai Publication No. Sho 60-112065 discloses a technology whereby a vibration prevention member is fixed to a frame portion. This technology is intended to prevent the noise from being produced from the display tube (especially from the frame portion). Also, Japanese Utility Model Application Kokai Publication Sho 62-1357 discloses a technology whereby a vibration prevention damper is provided at a filament portion. This technology is intended to prevent noise from occuring from the display tube (especially from the filament portion).
In the fluorescent character display tube, the filament supporting member vibrates in accordance with the filament driving frequency, and the resonance at the frame portion develops. When the vibration due to the resonance is transmitted through the frame out to the exterior, this becomes an unpleasant noise (the noise level being 30 dB to 60 dB). The use of the noise prevention member as disclosed in the Utility Model Application Kokai Publication No. Sho 60-112065 is effective for a certain one vibration frequency, but is not sufficiently effective in reducing noise levels for wider frequency ranges, and has suffered from an increase in the cost of materials and in the steps of manufacturing.
A further problem is that, when the tube assembly receives an external impact, the frame vibrates, the filament supporting member resonates and further the filament resonates. The filament that vibrates by the resonation may come into contact with the grid that is a control electrode, causing a material coated on the filament to be peeled off. The use of the filament vibration prevention technology disclosed in the Utility Model Application Kokai Publication No. Sho 62-2357 is not effective for the prevention of the peeling-off of the material coated on the filament since the contact of the filament onto the damper is unavoidable. The technology also suffers from an increase in the cost of materials and the steps of manufacturing, and from complicating the control electrode.